The hazards presented in the event of fire by large vessels containing pressurised fluids, particularly flammable fluids, such as railcars for the conveyance of liquefied petroleum gases, are well known. The pressure within such a vessel when subject to external heat, such as that due to an external fire, will rise, whilst the temperature of the walls of the vessels, particularly in areas not in contact with liquid contents of the vessel, may rise to a level at which their structural strength is sufficiently reduced to result in catastrophic failure to sustain the increased pressure within the vessel. Since such fires commonly arise following an accident such as a derailment, the orientation of the vessel when required to sustain such abnormal position is not predictable.
Whilst various proposals have been made with a view to reducing these hazards, the desirability of fitting such vessels with an effective pressure relief valve is widely recognized. The performance requirements for such a valve are onerous, since it must be capable of providing an adequate rate of fluid flow under a wide range of emergency conditions. It must also retain a reclosure capability such as to restrain unnecessary continuation of venting of the contents of a vessel when its internal pressure has fallen to an acceptable level, even after exposure to extreme temperatures. This acceptable pressure level should be substantially reduced in the case of a vessel which has been exposed to extreme temperatures to take account of the weakening of the vessel walls which may have occurred. According to circumstances, and the orientation of the vessel during venting, the valve may be required to pass gas, liquid, or a mixture of gas and liquid. For a given opening, the rate of volume discharge of gas will be much higher than that for liquid or liquid/gas mixtures. The valve must of course be gas and fluid tight under non-emergency conditions
Conventional pressure relief valves for this type of application have usually been of the poppet type, and rated for a desired vapour discharge rate. The configuration of a poppet valve is such as inherently to limit the valve opening which can be achieved within a valve body of given size, and the difference between opening and closing pressures is fixed by the design of the valve and generally quite small